1. Field of the Invention
This invention relates to seismic data processing and in particular to a method for attenuating the diffraction artifacts that are due to point-source scatterers that lie relatively close to the surface of the earth.
2. Discussion of the Related Art
In the art of geophysical seismic exploration, a plurality of seismic signal receivers are distributed over the earth along a line of survey at desired intervals commonly referred to as group intervals. An acoustic source occupies a series of positions, termed shot points, along the line, spaced-apart by some preferred multiple of the group interval. The source insonifies the earth at each shot point. A wavefield, emitted by the source, travels outwardly through the subsurface of the earth whence it is reflected or refracted back to the surface from various discontinuities which may be actual stratigraphic bedding planes or may simply be rough spots in the ground. The returning acoustic waves are detected by the receivers which convert the physical acoustic waves to electrical time-dependent seismic signals.
Seismic data processing, as is well known, provides means for converting the fleeting time-dependent seismic signals into time-scale cross section images for visually displaying some desired attribute of the subsurface of the earth such as its stratification. The accidental concomitant superposition on the cross section of seismic signals that represent an undesired attribute, is considered to be noise contamination. Acoustic studies are, of course, not confined to geophysical operations; they may also be applied to other arts, such as human tomography and non-destructive testing of structures, to which the teachings of this invention may be applicable.
Noise may be incoherent or random such as that due to wind, vehicular, pedestrian or bestial traffic on land. At sea, ship vibrations, surging of the hydrophone cable, flow noise of water around the cable and hydrophones, tugging of the cable by the tail buoy and marine life contribute to acoustic interference. Much of that sort of noise can be removed by destructive cancellation during routine stacking processes.
Noise also may be coherent. Assuming that primary reflection data from earth strata are the desiderata, coherent noise may comprise refractions, multiple reflections, reflected refractions or refracted reflections, diffractions from point sources on a rugose sea floor or from fault scarps, and coherent noise from mechanical sources such as the regular beating of a ship's screw. A particularly irksome noise source is the repetitive firing of a seismic acoustic source by a competing geophysical crew that may be many miles away. FIG. 1 is an example of severe coherent noise.
Some types of coherent noise can be removed by frequency filtering but only provided that the amplitude spectrum of the noise does not overlap the spectrum of the desired signal. Other well-known methods include spatial filtering, common offset or common midpoint stacking, vertical or lateral data averaging, array forming and beam steering. Velocity filtering is useful provided the velocity (i.e. the step-out time between adjacent traces, dx/dt) of the desired signal is different than the apparent velocity of the contaminating noise. However, it has been shown that energy diffracted from shallow point-sources may be actually enhanced when processed according to classical methods such as CMP stacking. See for example, Coherent Noise in Marine Seismic Data, by K. Larner et al., Geophysics, v. 48, n. 7, July, 1983.
U.S. Pat. No. 4,935,904 issued Jun. 19, 1990 to Chambers et al. discloses a method for suppressing coherent noise artifacts due to the abrupt termination of data processing at the lower boundary of a seismic recording. The boundary-effect artifacts are removed from the seismic recording by first extending the recording in time by padding it with zero data. The net effect is to push the boundary-effect artifacts below the time zone of interest. The extended recording is then clipped back to its original length in time, sans the pesky boundary-effect diffractions which now reside in the clipped-off portion of the recording.
Removal of coherent other-ship noise from a competing geophysical crew is taught by U.S. Pat. No. 4,937,794 issued Jun. 26, 1990 to Marschall et al. In this patent, the noise is removed by first rearranging the common shot gathers from normal shooting into common receiver gathers. Pairs of traces from a common receiver gather are corrected for differential moveout, weighted in inverse relation to the RMS signal power and combined to create compressed common receiver gathers. The compressed common receiver gathers may be reformatted to common midpoint gathers for further processing.
Another noise reduction method is taught by U.S. Pat. No. 4,866,679, issued Sep. 12, 1989 to R. Chambers for a Method for Identifying Anomalous Noise Amplitudes in Seismic Data. The method is accomplished by deriving a measured amplitude factor over predetermined time windows for a predetermined number of traces. A noise free amplitude factor for the same periods of time and number of traces is also computed using a Gauss-Seidel method of matrix inversion. A ratio of measured and noise free amplitude factors is made to determine a correction factor which is compared to a user-defined threshold value for determining the presence of anomalous noise amplitudes in the seismic records. Based on the comparison of the correction factor to the threshold value, the user determines whether to suppress or correct the traces experiencing the anomalous noise amplitudes.
A low-tech filtering process that sometimes works when more sophisticated techniques are temporarily unavailable, is the use of simulated anamorphic projection. To use that method on FIG. 1, for example, squint sideways across the Figure at an oblique angle. The reflected energy stands out at the expense of the noise. Reflections can be seen clearly from about 1.7 to nearly 4.0 seconds.
The above methods for suppressing coherent noise are special-purpose methods. There is a need for a more generally-applicable method for suppressing coherent noise that results from scatters that lie near the region where the seismic source and receivers are emplaced.